U.S. Pat. No. 1,275,507 issued August 1918 to Vuilleumier described a method of refrigeration based on heat pump technology. In very simple terms, Vuilleumier provided a one-phase refrigerator absorbing heat from a low temperature region and rejecting it to a higher temperature region. In practice, the Vuilleumier heat pump comprises a cold region, a hot region and a region of intermediate temperature, with a one-phase working fluid distributed throughout these three interconnected regions.
If the regions are regarded as cylinders for simplicity, the working fluid moves between a cold cylinder from which heat is withdrawn on one side of a first displacer to an intermediate cylinder to which heat is delivered on the other side of the first displacer. Similarly, the intermediate cylinder is separated from a hot cylinder in which the working fluid is driven between the hot cylinder on one side of a second displacer and the intermediate cylinder on the other side of the second displacer. The movement of the displacers changes the volumes of the different temperature regions relative to each other, with consequent changes in pressure. This drives the heat transfer. Although in theory two cycles of displacer operations are involved, in practice they are inter-dependent in an integrated device. The high and low temperature cylinders may be oriented at a 90.degree. angle to each other for preferred heat transforming conditions.
The coefficient of performance of the original Vuilleumier heat pump was not high and, although some modifications to it were introduced (see, for example, U.S. Pat. No. 2,127,286 issued 1935 to Bush and U.S. Pat. No. 2,567,454 issued September 1951 to Taconis), interest in it lapsed.
Historically, further development of the Vuilleumier heat pump and Stirling devices might have occurred if it had not been for advances in vapour compression refrigeration and developments such as the Linde, Claude and Heylandt cycles which have had enormous application for cryogenics. Also for cryogenics, the simple reversed Stirling engine is recognized. But all four of these require input of mechanical energy, whereas the Vuilleumier device requires very little mechanical input whether it is to be used for heating, cooling or both.
More recently, the Vuilleumier heat pump has been used for refrigeration in aeronautical and space applications, where its ability to produce very low temperatures with little mechanical complication is of greater importance than economy. Additional advantages are that the Vuilleumier device may be small, quiet and relatively maintenance-free. Recent concern about energy conservation and pollution of the environment has revived interest in the Vuilleumier device as a heat pump for residential heating and other purposes.
Attempts have, therefore, been made to improve the efficiency of the Vuilleumier device. Eder, in an article in the International Journal of Refrigeration (Vol. 5, No. 2, pp. 86-90, 1982) that was originally presented at the 11R meeting in Essen FRG in September 1981, discusses the performance of a Vuilleumier device. Nykyri and Hiismaki, in a report of Technical Research Centre of Finland No. 15/81, evaluate the Vuilleumier heat pump for heating applications. Nykyri and Hiismaki consider that the practical coefficient of performance for a Vuilleumier heat pump is 1.9 or even lower when heat losses are taken into consideration. They conclude that the main problem with the Vuilleumier Pump is the heat exchange at the low and intermediate temperatures.
The Vuilleumier concept is adaptable to various primary heat sources and, unlike conventional heat pumps, does not require electricity as its main power source, although a small amount of electricity or other power may be necessary to drive the displacers. Thus, even at a coefficient of performance of below 1.9, the Vuilleumier concept has its attractions because it is powered mainly by heat, and because energy losses incurred in the production of electricity to run a conventional heat pump need not be incurred. Even so, the achievable coefficient of performance of Vuilleumier devices and the efficiency of associated thermal regenerative machines such as the Stirling engine are not especially advantageous.